In Wild Fermentation most say "a couple of weeks" in the fridge.I assume a well constructed root cellar is about the same as a fridge, but what exactly happens when fermented products spoil?Is it that the bacteria run out of food and die off? Does it just get too sour to eat?Any good info? (I'm surprised at how little I could find online.)

I've left both sour pickles and kimchis in the fridge for months and months and a little over a year and they've been excellent after that time.

I've had pickled garlic in the fridge for two years and it's still getting better.

When I was maintaining a nuka pot (bucket) stuff was at room temp for days or weeks or months. And I'm not sure with that one where the line between good and spoiled was. Sometimes the things were so funky that I composted them instead of eating them. But nothing ever made me sick. And sometimes those funky things were great in soups where they mellowed.

Keep in mind that fermentation was most likely discovered and used as a method of preserving food. In fermentation pickling, the food itself produces the preservation agent, typically by a process that produces lactic acid. Fermented pickles include sauerkraut, nukazuke, kimchi, surströmming, and curtido. Preservation of food through lactic acid, alcohol, acetic acid and alkaline fermentations has been around since the dawn of human history. They must have been doing something right. ;)

But yes, your fermented foods should last a very long time as long as they are stored properly.

In alkaline-fermented foods, the protein of the raw materials is broken down into amino acids and peptides; ammonia is released during the fermentation, raising the pH of the final products and giving the food a strong ammoniacal smell. Most alkaline fermentations are achieved spontaneously by mixed bacteria cultures, principally dominated by Bacillus subtilis. In other cases, pure cultures can be used. For example, Japanese natto is inoculated with a pure culture of B. subtilis var natto. Pidan is a special example of alkaline fermentation made from fresh eggs. Instead of using microorganisms, pidan is made using an alkali-treated fermentation. Sodium hydroxide (NaOH) is produced from the reaction of sodium carbonate (Na2CO3), water (H2O), and calcium oxide (CaO) of pickle or coating mud. NaOH penetrates into the eggs, causing the physicochemical changes, color changes, and gelation. The appearance of pidan differs from fresh eggs in that the white becomes a semitransparent tea-brown color, and the yolk is solid or semisolid with a dark-green color. The nutritional value of pidan is slightly decreased compared with fresh eggs, but pidan has an extremely long shelf life and a pleasant, fragrant taste that is preferred by most people in Southeast Asian countries.

I've been culturing for a year or so with mixed results. Making small batches of all kinds of vegetable snacks and condiments so far... Still nervous, but getting over it. The back of my fridge has a lineup of various large mason jars of extra stuff made through the summer, and I just noticed the snap lids have popped to seal them-- they self-closed while waiting... . I'm fairly certain the lids were previously unused, and I know I didn't sterilize them... so there's an anaerobic situation there I'm trying to talk myself through....Yesterday I opened a jar of peas and herbs but was afraid to eat it, although it smells fine, if not interesting. Left it out overnight and this morning it was bubbling again.... Which is reassuring to the novice, and I feel fine about letting it rebrew before tasting.